getPDFSignatureCount: fix "too much recursion" error with some PDFs
The regex used to detect signatures uses "(.|\s)" to emulate dotall, since old browsers don't support it.
For some reason that leads to internal errors in Firefox with larger files in some cases.
Replacing it with a simpler "[\s\S]" class makes things work.
Also adds some tests with real PDF files, "QPDF-367-0.pdf" was triggering this issue.
Fixes #46 (closed)